1. Field of the Invention
The present invention is directed to systems for treating and re-cycling wellbore drilling cuttings, and, in one aspect, to such systems for offshore drilling rigs.
2. Description of Related Art
Drilling fluids used in hydrocarbon well drilling, as well known in the prior art, pick up solid cuttings and debris which must be removed if the fluid is to be re-used. Drilling fluid, called "mud," is typically either water based or oil-based. Typically a mud with various additives is pumped down through a hollow drill string (pipe, drill collar, bit, etc.) into a well being drilled and exits through holes in a drillbit. The mud picks up cuttings (rock), other solids, and various contaminants, such as, but not limited to, crude oil, water influx, salt and heavy metals from the well and carries them upwardly away from the bit and out of the well in a space between the well walls and the drill string. At the top of the well, the contaminated solids-laden mud is discharged over a shale shaker which has a series of screens that catch and remove solids from the mud as the mud passes through them. If drilled solids are not removed from the mud used during the drilling operation, recirculation of the drilled solids can create weight, viscosity, and gel problems in the mud, as well as increasing wear on mud pumps and other mechanical equipment used for drilling.
FIG. 3 discloses one example of a typical prior art system, land-based or offshore, (e.g. as shown in U.S. Pat. No. 5,190,645). A well is being drilled by a bit carried on a string of drill pipe as drilling mud is pumped by a pump into the drill pipe and out through nozzles in the bit. The mud cools and cleans the cutters of the bit and then passes up through the well annulus flushing cuttings out with it. After the mud is removed from the well annulus, it is treated before being pumped back into the pipe. The mud enters a shale shaker where the relatively large cuttings are removed. The mud then enters a degasser where gas can be removed if necessary. The degasser may be automatically turned on and off, as needed, in response to an electric or other suitable signal produced by a computer and communicated to degasser. The computer produces the signal as a function of data from a sensor assembly associated with shale shaker. The mud then passes to a desander and (or a desilter), for removal of smaller solids picked up in the well. In one aspect, the mud next passes to a treating station where, if necessary conditioning media, such as barite, may be added. Suitable flow controls e.g. a valve, control the flow of media. The valve may be automatically operated by an electric or other suitable signal produced by the computer as a function of the data from sensor assembly. From the treatment station, the mud is directed to a tank from which a pump takes suction, to be re-cycled through the well. The system shown is exemplary; additional components of the same types (e.g. additional treatment stations) or other types (e.g. centrifuges) are be included.
Remediation of cuttings on-site at an offshore rig is a difficult and expensive operation. It is known to have removed cuttings from a fluid in a barge to a land-based facility. This involves expensive and excessive handling which can result in a serious negative impact on the environment in the event of an accident or spill.
Several prior art systems which treat fluids or cuttings leave a relatively high oil content on the cuttings.
Thermal desorption processes are well known for remediating contaminated soil, both indirect processes in which material is isolated from flame and heat is applied above the vaporization temperature of a contaminant and direct process in which material is directly heated with a flame. Often in direct processes, VOC's are destroyed by direct flame contact and a portion of them may be thermally destroyed in a downstream oxidizer.
There has long been a need for an effective and efficient system for treating drilling fluids on offshore rigs. There has long been a need for such systems that produce an environmentally safe fluid which can be re-cycled without the need for treatment off site.